Abstract
Selectivity of food by L. rohita was studied in a stocking pond (Moat), by caleulating an Electivity index (E) for each food organism as described by Ivlev (1961). It was found that L. rohita was definitely selective in its feeding. In case of fingerlings, there was a strong selection for zooplanktonic organisms (Arcella and Difflugia among protozoans, Keratella and Brachionus among rotifer and Daphnia and Cyclops among crustaceans) and smaller algae (Cosmarium and Closterium among desmids, Euglena and Volvox among phytoflagellates and algal spores and zygotes) while most of the phytoplanktonic organisms, belonging to green algae, diatoms and blue green algae, were avoided. In case of adults, a strong negative selection was observed for all zooplanktonic organisms and a strong positive selection for most of the green algae and diatoms (Ankistrodesmus, Zygnema, Spirogyra, Selenastrum, Pediastrum, Scenedesmus, Tetraspora, Stephanodiscus, Naviculla, Diatoma, Synedra and Nitzchia). However, all blue green algae were avoided. The feeding relationship of L. rohita with other major carps, Cirrhina mrigala (Ham.) and Catla catla (Ham.) was studied in 2 different habitats, pond and river. It was observed that in both habitats all 3 spp were found to feed on almost similar types of food organisms, but the quantity of any food item eaten by adults differed markedly from one sp to another and the food items which were dominant in one sp, were of secondary importance for the other 2 spp. The adults of L. rohita were found to feed mainly on phytoplankton and macrovegetation, the main food of adult C. mrigala was decayed organic matter, sand and mud supplemented by plankton, while the food of adult C. catla was chiefly composed of zooplankton, and some phytoplankton. Hence there was no true identity of feeding habits between the adults of any 2 spp. However, there was an indication of competition for food between the fingerlings of all 3 spp, because all of them feed mainly on zooplankton (Crustaceans, rotifers and protozoans). However, such feeding habits lasted a very short time only and as the fishes grew, their feeding habits diverged.